Recently, as a liquid crystal display has had a large size and accordingly, the use of the liquid crystal display has been expanded from a personal use such as a mobile phone, a notebook, or the like, to a family use such as a wall-mountable TV, or the like. Therefore, high-definition, high-quality, and a wide viewing angle are required in the liquid crystal display. Particularly, since in a thin film transistor liquid crystal display (TFT-LCD) operated by a thin film transistor, each pixel is independently operated, a response time of a liquid crystal is significantly excellent, such that high quality moving pictures may be implemented, such that an application range thereof has been gradually expanded.
In order to use the liquid crystal as an optical switch in the TFT-LCD as described above, the liquid crystal should be initially aligned in a predetermined direction on a layer on which the innermost thin transistor of a display cell is formed. To this end, a liquid crystal alignment layer is used.
For the liquid crystal alignment as described above, in the past, a rubbing process of applying a polymer such as polyimide, or the like, on a transparent glass to form a polymer alignment layer and rubbing the alignment layer while rotating a rotation roller rolled with a rubbing cloth such as nylon, rayon, or the like, at a high speed to thereby align the liquid crystal has been applied.
However, recently, due to various problems of the rubbing process, a so-called “photo-alignment” of irradiating light such as ultraviolet (UV) light to align a predetermined polymer and align liquid crystal using the aligned polymer has more frequently studied and applied. In the photo-alignment as described above, a photosensitive group bonded to a predetermined photoreactive copolymer causes photoreactions by linearly polarized UV, and a main chain of the polymer is aligned in a predetermined direction during this process, such that the liquid crystals are aligned, thereby forming a photopolymerized liquid crystal alignment layer.
As a representative example of photo-alignment as described above, photo-alignment by photopolymerization was published by M. Schadt et al. (Jpn. J. Appl. Phys., Vo131., 1992, 2155), Dae S. Kang et al. (U.S. Pat. No. 5,464,669), and Yuriy Reznikov (Jpn. J. Appl. Phys. Vol. 34, 1995, L1000). As a photo-alignment polymer used in the above-mention Patent and articles, a polycinnamate-based polymer such as poly(vinyl cinnamate) (PVCN) or poly(vinyl methoxycinnamate) (PVMC) is mainly used.
In the case of photo-aligning the polycinnamate-based polymer, double bonds of cinnamate form a cyclobutane through a [2+2] cycloaddition reaction by the irradiated UV, such that anisotropy is formed, and thus liquid crystal molecules are aligned in one direction, thereby inducing alignment of the liquid crystal.
In addition, a polymer having a side chain including a photo-sensitive group such as a cinnamic acid group, or the like, in a main chain of acrylate, methacrylate, or the like, and an alignment layer including the polymer have been disclosed in Japanese Patent Publication No. 11-181127. In addition, the use of an alignment layer made of a polymethacrylic polymer has been disclosed in Korean Patent Laid-Open Publication No. 2002-006819.
However, the prior photo-alignment polymers for an alignment layer described above had disadvantages in that stability of the alignment layer was deteriorated due to low thermal stability of the polymer main chain, or photoreactivity, a liquid crystal alignment property or an alignment speed was not sufficient. In addition, since solubility of the existing photoreactive copolymers in an organic solvent was not high, there was a problem that processability of applying and drying a composition containing the photoreactive copolymer to form an alignment layer was deteriorated.
In addition, there is an application field requiring a change in anisotropy direction uniquely depending on a polarization direction, for example, a patterned retarder applied in order to implement three dimensional stereoscopic images, or the like, a patterned cell alignment layer, or the like. In the case of the existing photoreactive copolymers, if an alignment direction is determined once by polarized light, the direction does not move, or even though the direction moves, polarized light having a larger light intensity in a different direction is required.
Therefore, a photoreactive copolymer having excellent photoreactivity to various types of light, or the like, should be developed.